Greg Smith wrote: > After reading the code and that documentation a bit, the part I'm still > not sure about is whether the CLOG entry is created when the XID is > assigned and then kept current as the state changes, or whether that > isn't even in CLOG until the transaction is committed. It seems like the > latter, but there's some ambiguity in the wording and too many code paths > for me to map right now. pg_clog is allocated in pages of 8kB apiece(*). On allocation, pages are zeroed, which is the bit pattern for "transaction in progress". So when a transaction starts, it only needs to ensure that the pg_clog page that corresponds to it is allocated, but it need not write anything to it. (*) Each transaction needs 2 bits, so on a 8 kB page there is space for 4 transactions/byte * 8 pages * 1kB/page = 32k transactions. -- Alvaro Herrera http://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc.
